1. Field of the Invention
This invention relates to a process for modifying polytetrafluoroethylene and to modified polytetrafluoroethylene formed by the process. More particularly, this invention relates to a process for inducing high melting crystals in poly(tetrafluoroethylene).
2. Prior Art
Polytetrafluoroethylene exhibits useful properties over the widest temperature range of any known polymer. For a polymeric material polytetrafluoroethylene has a high virgin crystalline melting point of 342.degree. C. This reported value which is higher than other reported melting point values is due to super heating effects. Sintered or melt-crystallized polytetrafluoroethylene, which does not super heat as much, shows a melting point at about 327.degree. C. This equilibrium melting temperature (T.sub.m .degree.) i.e., the melting temperature of the most perfect crystals, is normally accepted as the melting point of poly(tetrafluoroethylene). B. Wunderlich, Macromolecular Physics-Crystal Melting, Vol. 3, pp. 48, 189 and 218, Academic Press, New York, New York (1980).
J. Grebowicz, "Thermal Properties of Drawn Polytetrafluoroethylene", Journal of Applied Polymer Science, Vol. 38, pp 707-716 (1989) describes certain experiments in which polytetrafluoroethylene is polymerized, drawn, sintered and annealed. It is reported that a high melting polytetrafluoroethylene having a melting point of 654K (381.degree. C.) was observed.